Sol-gel coatings, introduced in solid-phase microextraction (SPME) by Malik and coworkers in 1997 [1], have effectively overcome many of the drawbacks of fiber SPME [2]. Sol-gel capillary microextraction, also introduced by Malik and coworkers [3], effectively overcame the problems inherently associated with traditionally coated capillaries used for in-tube SPME [4], most notably limited thermal and solvent stability. The chemical anchoring of sol-gel coatings to the fiber [1] or inner walls of the fused silica capillary [3] is responsible for their enhanced thermal and solvent stability [1]. Sol-gel CME coatings have been effectively coupled with GC [3,5-10], capillary electrophoresis (CE) [11,12], inductively coupled plasma mass spectrometry [13,14], and high-performance liquid chromatography (HPLC) [15-21]. Sol-gel capillary microextraction coatings can easily be hyphenated on-line with HPLC [15-21], without the need to use complex desorption devices which are required to couple fiber SPME to HPLC [22]. The majority of sol-gel coatings are silica-based. Silica-based sol-gel coatings used in fiber SPME include sol-gel poly(dimethylsiloxane) (PDMS) [1], poly(ethylene glycol) (PEG) [23], calixarene [24], acrylate [25], hydroxy fullerene [26], and crown ether [27] coatings. Notable silica-based sol-gel coatings for capillary microextraction (CME) include sol-gel PDMS [3], sol-gel PEG [3,5], electrically charged sol-gels [11,12], sol-gel dendrimer [6], and sol-gel cyano-PDMS [7]. The most significant disadvantage of silica-based sol-gel coatings is the instability of the siloxane bond under acidic [28] and basic [29] conditions.
In 2007, Malik and coworkers [30] introduced the first germania-based sol-gel coatings for use in capillary microextraction. These coatings were used in conjunction with gas chromatography (GC). Like transition metal oxide-based sol-gel coatings, including sol-gel zirconia-[8], alumina-[31], and titania-[15,19,32,33] based coatings, the sol-gel germania coatings demonstrated enhanced pH stability [30]. However, the sol-gel germania-based coatings also offer the additional advantage of having similar surface chemistry to silica-based sol-gel coatings since germania is an isostructural analog of silica [34].